Over the past year, we have carried out crystallographic studies of catalytic intermediates supported by the catalytic site of Gia1. The structure of the GDP-bound complex has been determied at a series of Mg2+ concentrations in the presence of sulfate, an analog of inorganic phospate, a product of GTP hydrolysis. These crystal structures, determined with data obtained from the CHESS A1 beam line, demonstrate that restructuring of the switch II helix are concomitant with cleavage of the b-g phosphate bond of GTPgS, GDP and GDP[unreadable]Pi complexes using the A1 and F1 beamlines, reveaing the conformation of the protein in all three stable catalytic intermediate states. The complex between RGS4 (Regulator of G protein Signalling) and the GDP-Aluminum fluroride-Mg2+ bound form of Gia1, was also determined at 2.8[unreadable] resolution using radiation from the F2 line. This represents the first structure of a G protein-G protein GAP (GTPase Activating Protein) complex to be determined, and demonstrates, in part, how RGS4 stabilizes the transition state for GTP hydrolysis. Most recently, the structures of the stimulatory G protein alpha subunit Gsa and its complex with the soluble catalytic domains of its effector, adenylyl cyclase, have been determined. These structures, determined at 2.8 - 2.3[unreadable] resolutions (for the series of complexes studied) have revealed the mode of interacion between a heterotrimeric G protein and its effector, as well as the binding sites of ATP in the catalytic site of adenylyl cyclase and that of the diterpine activator, forskolin. These studies, enabled with data measured at the CHESS A1 beamlines, represent a milestone in structural studies of G proteins, and suggest for the first time, how heterotrimeric G proteins activate their effectors. Synchrotron radiation was essential to the success of all of the projects described above, due to the weak diffracing power and limiting size of the crystals utilized in each of the experiments